Processes for preparing cyclopentadienyliron (II) arene complexes rely on the ligand exchange reaction of eta-5 dicyclopentadienyliron (II) (commonly referred to as ferrocene) with an arene, in the presence of a Lewis acid. This reaction can be represented as: ##STR1##
where Cp represents a cyclopentadienyl anion, Ar represents an arene or arene anion, and L represents a Lewis acid.
This reaction reportedly involves removal of one cyclopentadienyl anion from ferrocene by the Lewis acid as described and whose structures are given in the Journal of Organometallic Chemistry Library 1977, 3, 311, and Tetrahedron 1983, 39, 4037. It is theorized that the reaction produces a coordinately unsaturated cyclopentadienyliron (II) cation and a cyclopentadienyl anion-Lewis acid complex (anion-acid complex). The cyclopentadienyliron (II) cation coordinates with the arene to give the cyclopentadienyliron (II) arene complex product. The cyclopentadienyl anion-Lewis acid complex can undergo further chemistry, the nature of which depends on the particular Lewis acid used. A commonly used Lewis acid for these reactions is aluminum chloride (AlCl.sub.3). Others include aluminum bromide, gallium chloride, zirconium tetrachloride, hafnium tetrachloride, boron trifluoride and tin tetrachloride. Mixtures of zirconium or hafnium tetrachloride with aluminum chloride and titanium tetrachloride have also been described (See EP-A 314,618 and U.S. Pat. No. 4,868,288).
In one prior art process of preparing cyclopentadienyliron (II) arene complexes, ferrocene is mixed with an arene, a Lewis acid, and a ferrous (Fe.sup.+2) salt, to produce a cyclopentadienyliron (II) arene cation complex. (See U.S. Pat. No. 5,059,701). The reaction mechanism is said to be as follows: the Lewis acid removes one cyclopentadienyl ligand from a ferrocene molecule resulting in a cyclopentadienyl anion-Lewis acid complex (anion-acid complex) plus a cyclopentadienyliron (II) cation; the cyclopentadienyliron (II) cation complexes with an arene to form the product cyclopentadienyliron (II)arene cation complex; the cyclopentadienyl anion-Lewis acid complex transfers a cyclopentadienyl anion to another ferrous ion to produce another cyclopentadienyliron (II) cation which can undergo complexation with another arene molecule to produce an additional cyclopentadienyliron (II) arene cation complex.
U.S. Pat. No. 5,059,701 specifically and expressly teaches the use of divalent (ferrous) iron (Fe.sup.++) ion as a reactant, as opposed to iron in any other oxidation state. The ferrous ion is said to react directly with the ferrocene. A described process of providing the ferrous ion is by reducing ferric ion to ferrous ion in a separate, preliminary processing step. The ferrous ion, produced beforehand in a separate step, is then added in the ferrous oxidation state to the other reactants (Lewis acid, ferrocene) to allow reaction of the ingredients toward the cyclopentadienyliron (II) arene complex.
It would be desirable, in the preparation of cyclopentadienyliron (II) arene complex compounds, instead of adding a source of ferrous ion to a reaction mixture, to add a source of ferric ion. This would allow, for one thing, the elimination of the separate, preliminary step described in the U.S. Pat. No. 5,059,701 patent of reducing the ferric ion to ferrous ion prior to reaction with the other reactants. It is described, however, in at least one previous teaching, that ferric ion (as compared to ferrous ion), when in the presence of ferrocene, reacts with the ferrocene to produce ferricenium tetrachloroferrate (III), [Cp.sub.2 Fe].sup.+ [FeCl.sub.4 ]. (See Boeyens et al., The reinvestigation of the ferrocene oxidation by iron (III) chloride in 2-butanone-ethanol, S. Afr. J. Chem. (1984) 37(1), 32-4). This teaching indicates that ferric ion added directly to reactants including ferrocene would result in production of ferricenium tetrachloroferrate (III), competing with the production of any other desired reaction product, such as a cyclopentadienyliron (II) arene cation complex.